Environmental remediation processes are useful in a wide variety of industrial applications, ranging from mining and coal applications to the treatment of groundwater, wastewater, and other industrial process streams. Regulations for controlling the discharge of industrial wastewater containing dissolved concentrations of heavy metals to the environment are being tightened due to concern for the presence of heavy metals in surface waters such as streams, rivers, and lakes. Heavy metal contaminants may include, for example, cadmium, chromium, copper, lead, mercury, nickel, zinc, and semi-metals such as arsenic and selenium. High levels of these metals in the environment can be detrimental to a variety of living species and ingestion of these metals by humans may cause accumulative poisoning, cancer, nervous system damage, and/or death. It is therefore desirable to treat wastewater to completely remove or reduce the amount of heavy metals to a safer level for both humans and animals prior to discharge into the environment.
Coal-fired power plants and waste incinerators may produce waste streams with high levels of heavy metals. Wastewater from power plants including flue gas desulfurization (FGD) systems may present a challenge due to the presence of mercury, selenium, and arsenic in the purge stream. Conventional treatment processes for removing heavy metals from water can be based on chemical precipitation and coagulation followed by conventional filtration, but these methods may not reduce metal concentrations to levels low enough to meet stringent drinking water standards.
Transition metal-based nanoparticles, such as zero-valent iron nanoparticles (ZVIN) and magnetite, have emerged as an alternative for environmental remediation due to their high surface area and high reactivity. Because transition metal-based nanoparticles possess various chemical properties derived from their different oxidation states, they have the ability to degrade a wide variety of toxic pollutants in soil and water, such as perchloroethene (PCE), trichloroethene (TCE), carbon tetrachloride (CT), nitrate, energetic munitions such as TNT and RDX, legacy organohalide pesticides such as lindane and DDT, as well as heavy metals such as chromium, lead, mercury, cadmium, and other inorganics such as selenium and arsenic. Processes employing transition metal-based nanoparticles may also provide cost savings as compared to conventional pump-and-treat or permeable reactive barrier methods.
Despite advances in transition metal-based remediation technology, such processes are not widely used in the industry due to several disadvantages, such as high operating costs, reuse and recovery difficulties, and/or aggregation effects on capacity and reactivity. These drawbacks can add complexity and cost to the overall remediation process. To address one or more of the disadvantages mentioned above, several methods have been developed to immobilize the transition metal-based nanoparticles on supports such as silica, sand, alumina, titania, and zeolite, to name a few. However, much like free-standing transition metal-based nanoparticles, these supports also require a follow-up filtration after use. Filtration methods such as membrane filtration reverse osmosis, electrodialysis reversal, and nanofiltration can be expensive and difficult to operate.
Moreover, the known methods for synthesizing transition metal-based nanoparticles, such as chemical vapor deposition, inert gas condensation, pulsed laser ablation, spark discharge generation, sputtering gas aggregation, thermal decomposition, thermal reduction of oxide compounds, hydrogenation of metallic complexes, and aqueous reduction of iron salts, tend to employ expensive reagents, produce large volumes of hydrogen gas, consume large amounts of energy, and/or cannot be scaled up for industrial application due to aggregation.
Carbothermal reduction methods may, for example, be used for the large scale production of various metals and alloys. For example, silicon, ferrosilicon, aluminum, iron, steel, and tungsten may be produced by reduction of metal oxides with a carbonaceous reducing agent in an electric arc furnace. Thermal energy is used to decompose the carbonaceous materials, which in turn drives the reduction of the metal oxide particles. The reaction is attractive as a scalable process because it is endothermic and yields only gaseous by-products. Carbothermal reduction methods may potentially be employed for the economical manufacture of transition metal-based nanoparticles. However, carbothermal methods for processing free-standing transition metal-based nanoparticles still suffer from other drawbacks mentioned above, and therefore do not offer a completely feasible solution for the production of transition metal-based nanoparticles.
Accordingly, it would be advantageous to provide an efficient, cost-effective, easily operable, and/or scalable process for making transition metal-based nanoparticles. The resulting transition metal-based nanoparticles can be used in a wide variety of environmental remediation applications, such as groundwater and wastewater treatment.